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Fiber Basics
EMS Name used to refer to a range of different types or forms of radiation.
*We look at these different forms of radiation as waves.
*What makes each region different from the other is the wavelength.
*the shorter the wavelength the higher the frequency, the longer the wavelength
the lower the frequency
*white light consists of many different electromagnetic waves
*All electromagnetic waves travel at the speed of light (180,000m/s)
*A given wavelength travel at different speeds in different materials (for a given
material different wavelengths will travel at different speeds in that material)
*The length of a wave indicates its colour
*important thing in fiber optics is to have total internal reflection. Once there is
refraction at a much low level there maybe wave refraction outside the cladding
affecting the signal as well as the bandwidth (attenuation).
Efficient wavelengths which exist in the infrared region
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850nm, 1300nm, 1310nm, 1550nm
As frequency of a wavelength increases, the wavelength gets shorter, so that it can
occur more often in the same period of time.
As we are at a wavelength lower than 850nm we have high attenuation, as we
approach 850nm the attenuation decreases, passing 850nm attenuation rises again
to a peak then decreases once again as it approaches 1300nm and continues to
1550nm. (**view dotted line**)
*850nm & 1300nm are associated with the use of multimode fiber
*1310nm & 1550nm are associated with the use of single mode fiber
Spectral width the amount of wavelengths for a given light source.
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*No light source emits a single wavelength
LIGHT SOURCES
Light emitting diode (LED)
Low cost
Low power
Wide spectral width
Laser Diode (less prism effect taking place separation of the different
wavelengths)
High cost
Medium power
Narrow spectral width
Table 1 - Comparison of LEDs and Lasers
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Characteristic LEDs Lasers
Output PowerLinearly proportional to drive
currentProportional to current above the
threshold
Current Drive Current: 50 to 100 mA Peak Threshold Current: 5 to 40 mA
Coupled Power Moderate High
Speed Slower Faster
Output Pattern Higher Lower
Bandwidth Moderate High
Wavelengths Available 0.66 to 1.65 m 0.78 to 1.65 m
Spectral Width Wider (40-190 nm FWHM)Narrower (0.00001 nm to 10 nm
FWHM)
Fiber Type Multimode Only SM, MM
Ease of Use Easier Harder
Lifetime Longer Long
Cost Low ($5-$300) High ($100-$10,000)
Critical angle angle that exist at the point where total internal reflection begins to
take place
Numerical aperture NA an indication of how many light at different angles will
accommodate total internal reflection (light at different angles
larger aperture accommodates more angles at which the light reflects
smaller aperture accommodates less angles
for acceptance light angle must at least be equal or runs parallel to the
acceptance angle or run closer to the path of the core
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A single fiber can provide multiple paths for light travel
These paths are called modes
Over 100,000 modes can exist
Material make-up of fibers
Ultra pure, ultra transparent silicon dioxide or fused quartz
Impurities are added during manufg. to create the required refractive index
Refractive index profile There are two types
Step index profile There is a sudden change in the refractive index value
when you look at the core related to cladding. (like from air to glass)
Graded index profile There is no longer a sudden change of refractive
index between the core and the cladding, a gradual changes takes place.
Note the core has a higher refractive index value than the cladding
Lot simpler to create step index fiber
Step index multimode was 1st fiber designed but is too slow for most uses due to the
dispersion caused by the different path lengths of the various modes
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Problems with the efficient transmission of light
Pulse spreading (Dispersion) Affects the bandwidth (limits bandwidth)
Modal dispersion is a bandwidth limiter for multimode fiber, happens in
single mode but has no great impact.
Chromatic dispersion affect bandwidth in single mode fiber
Attenuation Affects the power/strength of the signal (weakens signal)
The decrease in the power of an optical signal from input to output (loss of
light measured in decibels dB)
Example 3dB = 50% loss of light
10dB = 90% loss
20dB = 99% loss
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Typical Attenuation - Loss of light / optical power loss
Fiber type 850nm 1310nm 1550nm
Single mode NA 0.35 dB/Km 0.25 dB/Km
Multimode 3.5 dB/Km 1.0 dB/Km NA
Factors that cause attenuation
Intrinsic causes Internal to the fiber
Absorption - longer wavelength less absorption, shorter wavelength more
absorption
Scattering - major cause of attenuation. (In multimode fiber)
*Note some light will scatter and enter back to the source interfering with the signal
transmitted.
Extrinsic causes external to fiber
Micro-bending pressure on the fiber
Macro-bending bending the fiber more than specified messing with the angles
resulting in refraction
Fiber curl has to do with spicing (one end curls resulting in fibers not being aligned
correctly)
Core concentricity how centered is the core in the entire fiber
Ellipticity How round the core is
Structural Elements & Functions
Buffer Tube
Strength members Kevlar (Aramid Yarn)
Outer jacket
Inner jacket
Gel Filling Compound used for high vibration type environment
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Water Blocking Compound Gel found outside of the buffer tube to assist in keeping
water out
Binding tape or yarn to keep the assembly together
Armour
Ribbon referring directly to the fiber itself
BUFFER TUBE DESIGNS
2 types of buffer tube leads to 7 designs, 5 lose tube designs and 2 tight tube
designs
Loose tube design
Single fiber per tube
Multiple fibers per tube
Central buffer tube
Ribbon
Start or slotted
Tight tube
Break out (each fiber has its own kevlar)
Distribution (premise)
Cable types: Zipcord, Distribution, Loose tube, Break out
Tight Buffer cables can be used for indoors as well as outdoor.
Typical components
900um buffer fiber
Fibers are additionally buffered
Tight bound design
Core locked design
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Fiber glass central strength members
Kevlar strength members
Inner/outer jackets
No Gel
Variation & improvement to this cable
Typical components
Individual fibers break out like a patch cord
Flexible non-memory breakout
Kevlar strength members
Amoured jackets
Central tube only style
Quick to break out
Pull in by standard mesh grip
More flexible to pull in
Adv of loose tube over tight tube
Loose tube is made for the outdoor environment
Proven for long outdoor runs with wide temp range
Less expensive above 24-fiber count cables
Have better packing density
Available in armored for direct buried. All dielectric for aerial and ducted
applications & riser rated construction for use in riser applications
Note loose tube cables shouldnt be carried into a building more than 50
feet, due to electrical standards, has materials that could be toxic
250m found in the loose tube type cables & cannot be terminated directly
with connector
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Adv of tight buffered/tube over loose tube
Can be used in intra-building backbone, Horizontal distribution, for patch cords &
equipment cables
Increased physical flex
Smaller bend radius for lower fiber count cables
Easier handling characteristics in low fiber count
Fiber ends can be terminated directly with connectors (900m found in these
Tight buffer)
Important performance specs. Installation specs
Maximum recommended installation load (pulling force being exerted on the
cable)
Minimum recommended short-term (during installation) bend radius
Minimum recommended long term (after installation, at rest) bend radius
Temp ranges (both high n low limits) for installation and temp ranges when being
stored
Moisture/water resistance how much pressure under water the cable canwithstand before its resistance is breached.
Cable diameter if running in ducts already containing cables the size of the fiber
needs to be known to match the space remaining in that duct.
Important performance specs. Environmental specs
Temperature range of operation
Compliance with NEC (national electrical code) or local electrical codes
Radiation resistance
Vertical rise distance cable has weight the further you go upwards the more
pressure on the cable at the farthest end
Flame resistance how well the cable resist fire
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Crush loads
Toxicity should not give off a certain amount of toxic fumes for a given period of
time
Vibration
In fiber technology splicing is the continuity of total internal reflection
Must have:
Precise alignment
Fiber retention ensures whatever clamping mechanism used will properly restrain
the fiber over a period of time
End face protection the use of dust caps
Splices
Permanent or semi-permanentmechanical spice
Used when rerouting of optical path is not required or expected
Mid-span splices- connects 2 lengths of fiber (with a third in the middle)
Eg ______1___________3_____________2_______
Pigtail splicing used at the fiber ends length of cable or fiber terminated at oneend only
The other end of the pig tail would be connected (spliced) to the oncoming
cable
Connection Loss Factors
End-face quality
Polishing clean fiber core, no cracks, scratches, pits
Cleaving:
Good Cleave smooth, mirrored surface
Bad Cleave chips and shards
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Preparation for fusion splicing:
1. Remove the coating
2. Cleave/Cut to proper lengths
3. Place in the fusion device. Align & Gap
4. The fiber cores are melted together. 2 ends are now fused together
5. And heat-shrunk
Fusion splicing vs Mechanical splicing
Fusion splicing Mechanical Splicing
Capital Investment Expensive Cost effective
Tech knowledge/training Very high low
Splicing time Long Quick
Construction applications Yes Yes
Maintenance application Yes Yes
Affected by environment Yes No
Requires power source Yes No
Air condition required Yes No
Attenuation
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*Note for patch cables Yellow signifies single mode while Orange signifies
multimode
Connectors 5 basic features
Method of coupling
Keying can only be plugged in, in one particular way
Contact of fiber cores
Style SC, ST, etc
Installation technique
Connectors 3 Part connector system
Connector or plug
Receptacle
Adapter
Connector installation methods 4 General types
Method 1 Epoxy, Crimp & Polish (Epoxy)
Method2 preloaded, Preheated & Polish (3M Hot Melt Proprietary)
Method3 Epoxyless, Crimp/Crimp/Polish (Mechanical)
Method4 Anaerobic Adhesive, Crimp & Polish
Variation of Method 3 No Polish (Cleave & Polish)
Attenuators Variable & Fixed
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Fiber optic systems
Can be grouped into 4 categories
Point-to-point
Point-to-multipoint
Network
Switched network
Point-to-point
Simplest form
2-way communication between systems
Each end has permanent linked TX and Receiver Pair
Distance is of little or no concern
Concept can be imposed on different types of transmission systems
Point-to-Multipoint
Signals sent from 1 TX to many terminals
Sometimes called broadcasting
Terminals may or may not send signals to transmitter
If they do, the return signals are often at a lower speed than the broadcast signal
Fixed with permanent connections
Typically include multiple levels of signal distribution
If the TX dies, the system dies
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Network
2-way transmission between any pair or terminals
Different physical topologies exist: Ring, Bus, Star, Mesh, etc
Permanent connection to each terminal
Switched
Creates flexibility
Allows any pair of terminals to send/receive signals directly to/from each other
Adds a level of complexity that makes and breaks connection
*note. In practice, most switching is performed electronically with fiber optic point
to point systems linking electronic switches
It is difficult to switch optically
Cost is a factor